The present invention relates to laser scanner systems, and, more particularly, to laser scanner systems of the holographic type.
The advantages of using holographic deflection technology for laser scanner systems rather than rotating polygons or galvanometer-driven mirrors are well-known. Implementation of holographic systems, at present, has several limitations. First, the maximum system efficiency attainable demands either (a) that so-called Bragg holograms be used which can have nearly one hundred percent (100%) internal efficiency (Bragg holograms are fragile and not amenable to replication), or (b) that deep-groove holograms be used which can approach nearly one hundred percent (100%) internal efficiency. In addition, the type of hologram chosen can introduce optical aberrations and distortion in the system; for example, (a) so-called Seidel aberrations and (b) line bow.
As is well-known, hologram efficiency is polarization-dependent. Present day holographic scanners therefore use polarized lasers with the polarization orientation determined by hologram fringe direction. A scanner using a hologram dependent on a plane polarized beam cannot be used to read paper documents, since the light reflected from a diffusing object, such as paper, would not retain its polarization. Diffusely reflected light, even when the source is polarized, has random polarization. The polarization directions can be resolved into so-called p and s components and the efficiency calculated for each meridian. To maximize efficiency for randomly polarized light, the hologram must have substantially equal s and p plane terms.
A scanner has been described in U.S. Pat. No. 4,239,326 (Kramer) as one which uses an array of linear sine wave grating segments recorded holographically on a disk. However, in such scanner, there is a dependency of efficiency on p-plane polarization. The use of this configuration has two disadvantages: (a) the scanned line is not straight, but has a bow dependent on small changes in the incident angle; and (b) the high diffraction efficiency is obtained for only one polarization direction, so that if an attempt is made to use such a scanner to read a diffuse surface, such as a sheet of paper, the polarization is lost and the efficiency is correspondingly lowered. Thus, a scanner incorporating the above holographic elements can be used for a recording scanner, but not for reading through common optics.
Another scanner system is shown in U.S. Pat. No. 3,953,105 (Darien) in which an optical spinner is used to direct coherent light toward a scanning focal point locus. In such system, however, an auxiliary reflective element, such as a spherical mirror, must be used, thereby increasing system cost and alignment difficulty.
It is, accordingly, a primary object of the present invention to provide a scanner system having immunity from polarization dependence, having improved line bow correction and which reduces image aberrations.